Fuel cell power systems have an advantage over batteries in that they can be readily refuelable, and therefore a combination of a “replaceable” fuel cartridge and a “permanent” module can allow extended runtime operations without the need for grid electricity for recharging.
Although hydrogen is the fuel of choice for fuel cells, widespread use is complicated by the difficulties in storing the gaseous hydrogen. Many hydrogen carriers, including hydrocarbons, metal hydrides, and chemical hydrides are being considered as hydrogen storage and supply systems for generation of hydrogen on demand, e.g., by reformation from hydrocarbons, desorption from metal hydrides, or catalyzed hydrolysis from metal hydrides and water. Preferably the fuel mixture has a high gravimetric energy density, and controllable hydrogen generation rate, i.e., flow rate and pressure may be controlled to meet demands of a fuel cell.
Reformable fuels, which are typically defined as any substantially liquid or flowable fuel material that can be converted to hydrogen via a chemical reaction known as reformation, including for example hydrocarbons, and chemical hydrides, produce hydrogen and other gaseous and non-gaseous products. For hydrocarbons, the non-hydrogen by-products comprise carbon oxides, e.g., CO2 and CO, and potentially other gaseous products. The resulting hydrogen rich gaseous product stream is typically sent through a purification stream before being sent to, e.g., a fuel cell unit. Hydrocarbon fuels useful for fuel cartridge systems include, for example, methanol, ethanol, methane, propane, butane, gasoline, and diesel fuel. As an example, methanol is a preferred fuel which reacts with water to form hydrogen and carbon dioxide.CH3OH+H20→3H2+CO2  Equation 1
One of the more promising systems for hydrogen storage and generation utilizes borohydride compounds as hydrogen storage media. Such compounds react with water to produce hydrogen gas and a borate in accordance with the following simplified hydrolysis reaction:MBH4+2H2O→MBO2+4H2+300 kJ  Equation 2where MBH4 and MBO2, respectively, represent a metal borohydride and a metal metaborate. In practice, the borate is actually in one or more hydrated states, e.g., tetrahydrate, dehydrate, or hemihydrate. The rate of decomposition of the metal borohydride into hydrogen gas and a metal metaborate is pH dependent, with higher pH values hindering the hydrolysis. Accordingly, a stabilizer, such as an alkali metal hydroxide is typically added to solutions of a complex metal hydride in water to be used as the fuel from which the hydrogen gas is generated. Heat or a catalyst, e.g. acids or a variety of transition metals, can be used to accelerate the hydrolysis reaction.
Sodium borohydride (NaBH4) is of particular interest because it can be dissolved in alkaline water solutions with virtually no reaction; in this case, the stabilized alkaline solution of sodium borohydride is referred to as fuel. Furthermore, the aqueous borohydride fuel solutions are non-volatile and will not burn. This imparts handling and transport ease both in the bulk sense and within the hydrogen generator itself.
Various hydrogen generation systems have been developed for the production of hydrogen gas from aqueous sodium borohydride fuel solutions. The advantage of such borohydride hydrogen generation systems is that they can be scaled to feed fuel cells of power ranges from less than 10 watts to greater than 50 kilowatts. In most cases, it is preferred that hydrogen generation systems be efficient and compact, have a high gravimetric hydrogen storage density, and are readily controllable to match hydrogen flow rate and pressure to the operating demands of the fuel cell. The challenge in designing such systems is to maximize energy density by minimizing the associated balance of plant components to reduce volume, weight, parasitic load and general system complexity.
A simple conventional system (FIG. 1) for generating hydrogen on demand comprises a fuel reservoir, fuel lines and a pump for delivering fuel to a reaction zone, or reaction chamber, which may contain a catalyst, and outlets for separation of gaseous hydrogen and other reaction products. However, when there is a requirement for additional fuel components, such as when mixing two or more fuel components of a mixture, or diluting a concentrated fuel mixture, a more complex system is required with additional pumps and flow controllers or fuel regulators.
For example, a system for generating hydrogen from solid and liquid fuel components has been described in U.S. patent application Ser. No. 10/115,269, filed Apr. 2, 2002, now U.S. Pat. No. 7,282,073 entitled “Method and System for Generating Hydrogen by Dispensing Solid and Liquid Fuel Components,” which is commonly assigned. Such systems utilize separate dispensing and delivery mechanisms for each fuel component.
Hydrogen generation systems may recycle or recover reaction products to control the reaction or to increase efficiency of conversion. For example, the reactant may be withdrawn from the reaction chamber to stop the reaction as described in described in U.S. Pat. No. 6,534,033 entitled “System for Hydrogen Generation,” which is commonly assigned, where, in a process for generating hydrogen from a stabilized metal hydride solution, a reversible fuel pump is in fluid communication with a fuel solution reservoir and a reaction chamber containing a hydrogen generation catalyst. The pump can run in a forward direction to deliver fuel to the reaction chamber and then in a reverse direction to drain the reaction chamber to stop hydrogen generation.
Clogging by precipitation of solid reactants from reactant solutions or precipitations of reactants or reaction products in pumps and valves may be a significant issue. Various approaches are known to allow for controlling the reaction chemistry, or flushing of the system with water or other diluent to reduce clogging. Some systems recycle fuel to increase the efficiency of hydrogen generation. It is preferable in other systems that solid by-products, and fluid reaction products which may precipitate out, are not recycled back to the reaction chamber or the fuel reservoir, to avoid clogging. However, since water is generated in significant quantities as a reaction product in hydrogen fuel cells, it may be recycled into the fuel mixture as a diluent, or used for flushing the system. Such a system which provides for water to be recovered from the exhaust of a fuel cell or condensed from a hydrogen gas stream is described for example in U.S. patent application Ser. No. 10/223,871, now U.S. Pat. No. 7,803,657, entitled “System for hydrogen generation,” which is commonly assigned.
Since gravimetric energy density is one of the key factors affecting the cost of hydrogen generation technology, it is desirable to provide a more concentrated fuel solution and a diluent, or multi-component fuel mixtures, which may be stored in concentrated form and mixed or diluted on demand (e.g., hydride and water or other aqueous reactant). Nevertheless, additional pumps required for additional components are a significant cost in dollars and energy density. Pumps are the active mechanical component that are most likely to break down, particularly if clogging is an issue, thus affecting reliability. Thus, current systems have limitations and alternative systems and methods with improved energy density, cost and reliability are required for systems for hydrogen generation on large and small scale when using multi-component fuel mixtures, or for mixing recycled or recovered fluid reaction products with fuel components.